This invention relates to a method for the fabrication of a direct-heated cathode structure and an apparatus which is suitable to carry out the method, and more particularly to a newly improved method for the fabrication and an apparatus so prepared to properly fabricate a direct-heated cathode structure which include a filament that heat its cathode pellet and absorb the deformation by heat expansion for itself.
While advantages of a direct-heated cathode structure are its low power consumption rate and quick starting characteristic because its heat loss is lessened and its starting time is shortened by placing its cathode pellet on the filament directly, disadvanages thereof are in that heated filament cause tension changes in itself with change of disposition interspace between the first grid and cathode pellet concurred.
These disadvantages affect on cut off voltage during the operation of cathode-ray tube thus deteriorating white balance.
To solve this problem, a structure so constructed that springs supporting the filament at its both ends may absorb the thermal expansion is employed hitherto, however this kind of structure can hardly be miniaturized and it needs a plurality of fabrication processes thus causing productivity in the fabrication of the direct-heated cathode structure difficult to be enhanced.
Different from the above mentioned direct-heated cathode structure, another one disclosed in U.S. Pat. No. 4,195,246 have its both ends of filament bended so as to render the filament itself absorb the tension change originated from thermal expansion.
This manner of solution to the present problem suggested a remarkable structure for direct-heated cathode structures, however it is somewhat dissatisfying in respect to the miniaturization of the structure because of its filament bended across the direction that link upper ends of two lead bars.
A direct-heated cathode structure with a corrugate-ended filament that not only itself function as a spring but also accomplished the miniaturization in its volume by getting its end bent in a zigzag pattern is proposed recently and employed for electric view finders which need subminiature electron guns.
However, difficulties which come across during the fabrication of this kind of direct-heated cathode structure are caused by the increased number of processes which the filament have to undergo through so as to have its ends corrugated and in a welding process wherein the corrugated filament is connected to lead bars.
To put it rather concretely, it can be said that we have to undergo through a plurality of pressing processes because corrugate bending of the filament must yield a homogeneous product that is free from residual stresses arisen by pressing, and we have to bear with the corresponding difficulty in electric resistance welding because the filament and lead bars connected to it are of thin and narrow strip shapes.
Therefore this kind of directed-heated cathode structure confronts problems that: the number of processes are increased and equipments relevant to these processes become oversized because it have to undergo through pressing processes of many steps, and a jig of complicated structure is required to fix the tungsten filament on lead bars temporarily to enable the welding.